This invention relates to a process for manufacturing welded tubes and pipes (hereinafter collectively referred to as "welded tubes") from an inexpensive titanium alloy having improved resistance to crevice corrosion and to acids. More particularly, it relates to a process for manufacturing welded titanium alloy tubes having improved corrosion resistance in environments inducing severe crevice corrosion or in non-oxidizing acid environments, which pure titanium metal can no longer withstand.
Titanium has good corrosion resistance in sea water and in oxidizing acids such as nitric acid and it is widely used as a material for condensers in nuclear power stations and heat-exchanger tubes in chemical plants. However, its resistance to crevice corrosion is poor in high-temperature corrosive environments containing chloride ions. Therefore, titanium alloys containing 0.12%-0.25% by weight of palladium (Ti--0.12/0.25Pd) as specified in ASTM grade 7 or 11 (or JIS Classes 11 to 13) are recommended for use in such environments. The use of these alloys which contain expensive Pd metal in a relatively large amount is limited due to their high costs.
An attempt has been made to develop a more economical titanium alloy having resistance to crevice corrosion. Japanese Unexamined Patent Application Kokai Nos. 62-107041(1987), 62-149836(1987), 64-21040(1989), and 64-21041(1989) disclose corrosion-resistant titanium alloys which contain relatively small amounts of one or more of the platinum group metals, one or two of Ni and Co, and optionally one or more of Mo, W, and V.
In order to apply these titanium alloys to actual products, a commercial manufacturing process of the products should be established so as to make it possible to efficiently manufacture products having optimum properties. This is important since the properties of titanium and titanium alloys significantly vary depending on the manufacturing process and conditions, especially working and heating conditions.
Particularly in the manufacture of welded tubes, such as for use in heat exchangers, it is impossible to provide a product having both good mechanical properties and good corrosion resistance unless all the steps from the fabrication of a slab and a hot-rolled or cold-rolled coil or strip to final heat treatment are performed under properly controlled conditions. However, the optimal conditions for the manufacture of welded titanium alloy tubes have not been investigated sufficiently in the past. Thus, there is a need to establish a process and conditions for the commercial manufacture of corrosion-resistant welded titanium alloy tubes of good quality.